Radio frequency impedance meter



Dec. 20, 1955 J. H. PRIEDIGKEIT RADIO FREQUE'NCY IMPEDANCE METER 2 Sheets-Sheet l Filed Sept. 4, 1951 INVENTOR PRIEDIGKEIT JOHN ATTOR N E 313 Dec. 20, 1955 J. H. PRIEDIGKEIT RADIO FREQUENCY IMPEDANCE METER 2 Sheets-Sheet 2 Filed Sept. 4, 1951 I.IIIIIIIIIIIIIIII JOHN H. PRIEDIGKEIT ATTORNEYS Um'ted States Patent O 2,728,048 RADIO FREQUENCY IMPEDANCE METER John H. Priedigkeit, Berkeley, Calif., assignor to the United States of America as represented by the Seeretary of the Navy Application September 4, 1951, Serial N0. 244,911 6 Claims. (C1. 32457) Fig. l is a circuit dmgram cf a preferred embodiment of Ih8 features cf the ptesent invention; and

Fig. 2 is a circi1it diagram of a variant embodiment of the fetures Cf the present"invention.

Referring noW to Fx'g. 1 of the drawxngs, there is shown oscillator'section 11, a completely shielded thermocouple sect1'on 13, and an oscillator power cc' mrol section 14.

-The oscillator 11 employs a triode tube 15 connected Bator circuit with the plate bypassed to ot capacitance 16. The circuit employs a tapped inductance 17 which 1's shown with one end 18 thereof connected to ground and the opposing end 19 2,728048 Patented Dec. 20, 1955 connected to the grid 20 cf tube cuit.

may compn'se several separate inductors, by means not shown, to provide several frequency of operation.

bands to the gn'd 25 of tube 26 which is connected as a cathode-follower ampli- Th1's cathode-follower circui} includes also cathode thk: cathode of tube 26.

The cathode of tube 26 1's connected by way of capacitance 31, fuse 32 and thermocouple 33 to the output jack 34. In operation the unknown in1pedance to be measured 1's connected between v Tube 36 1's inserted in seris in the plate gupply lead for tube 15. Tube 36 is connected somewhat as a cathode-follower havmg its anode 37 ditential selected by potentiometer 43.

The vacuum tube voltmeter section 12 of the apparatus employs a pair of diode type electron tubes 47 and 48 connected in a rectifymg and balancing circuit. Tube 47 has its anode 49 connected to jack 34 through a small coupling capacitance 50, and therefore develops a D.-C.

' and also preferably provicle a tirne constant circuit for filtering the rectified voltage. A thermionic diode employed in such a rectification circuit has several limitations. It does not operate merely a s a unilateral impedance device but also acts somewhat as a low voltage battery due to such efiects as ernission voltage and contact otential. T permit compensation for ihis. the balancing tube 48 is connected in a circuit which is identical to that of tube 47 With the exception that the anode thereof does not receive a radio-frequency voltage. The theory behind this is that the D.C. voltage developed bythermionic action in tube 48 and ootained across capacitance 53 will be equal to that developed in tube 47 and obtained across capacitanee 51 so that oy subtracting one D.-C Potential irom the other, the net efleet will be zero.

Such sub traction or combination in opposition is provided in the push-pull two'-stage D.-C. amplifier of tubes 54, '55, 67 and 68. Connection of the grid of tube 54 is made to capacitance 51 through a three-position switc h 58; and the grid of tube 55 is connected to capacitance 53 through the three position switch 59 which is ganged t'o switch 58'. Switches 58 and 59 provide ste9wise selection of the percentage of voltage from capacitances 51 and 53 pplied to the grids of tubes 54 and 55 so that several voltage ranges may be accornrnodated. In one position of switches 58 and 59, the grids of t bes 54 and 55 are grounded to facilitate balancing the D.-C. amplifiel System. In a typical embodirnent the rnid-position of switches 58 and 59 provides for a fifteen-volt measurement Whereas the upper position (full voltaige) provides for a four and one half volt measurement.

Additionally ir1terposed in the signal paths between the gxids of tubes 54 211d 55 aridthe capac itances 51 and 53 are balancing otentiometers 60 and 61 which provide a means of adjusting for unbalance between the tubes 47 and 48.

.In the D.-C. amplifier, tubes 54 and 55 have their cath0des mutually connected to ground thr ough resisb ance 62. This assists in the introduction of the compensting signal from tube 48 because biasing voltage deireloped across resistance 62 will cause conductivity to cha.nge inversely in tubes 54 and 55 responsive to a change at the grid of either tube. y

T\1bes 54 and 55 f0rrning a pushqmll voltage amplifier have anode load resis tances 63 and 64 which are q0nne oted to opposing ends of otentiometer ;65, the eenterl tap 66 t hereof leading vto a source of regulated (B+). Potentiometer 65 provides a further means 015 balancing the D.- C. amplifier.

The anodes oftubes 54'and 55 are directly connected to the grids of cathode-follower tubes 67 and 68 whieh have separate cathocle resistors 69, 70. Thus .by well known Cathode-follower action, cathode voltages, obtained at low impedance, are always quite close to the voltages of the anodes of the tubes 54 and 55. 'I'he cathodes of tubes 67 and 68 are connected together through a series path including resistance 71 and milliammeter 7810 prevent damage to the meter. This series pathwill be non-conduotive whenever the cathodes of tubes 67 and 68 are at similar otentials; however, conduction therethrough will take place whenever a potential difierence exists between the cathodes. Resistance 71 must be adjusted in Operation to lirnit the maximum (3111133111 flow through'ineter 78.

In adjustment of the circuit blancingthe voltrnetoeris the first step. With the switches 58 and 59 in their lowest position so that the grids of tubes 54 and 55'211'e grounded, otentiometer 65 is. adjusted to where' there is 'no current flow indicated by-meter 78. With the oscillator .tube inoperative, rendered so by rnkxving the tap of otentiometer 43 to the ground end thereof with jack 34 grounded 10 assure thatv no radio irequenoy voltage is applied to diode4'l, the swit ches 58-59 are moved to thei1' top osition, eonnecting the grids of tubes 54 and direct to the taps cf otentiometers and 61. potentiometers 60 and 61 are then adjust ed to where the meter 78 also indicates zero current flow. This adjustment is preferably completed with the tap 011 one of the potentiometers at the upper end of the potentiometer so tnat the grid of one of thetubes 54 01 55 is in efiect connected directly across the corresponding capaeitance 51 er 53. In practice it will be found that it is as lllrely to be one potentiometer as the other dependix1g upon which o'r' the diodes 47 or 48 'develops the higher contact potential. Normally there will be- Very little'difference so that for all practical purposes the grids of both tubes 54 and 55 will be diree ly placed across capacitances 51 and 53.

With the vacuum tobe voltmeter 12 thus balanced it may be desirable to check and set the thermocouple 33 and its meter 35. T0 d0 this an external.battery in series with a DC. milliarnmeter may be connected between jack 34 and ground. The meter 35 may ihen be adjusted so that its reading corresponds to the reading of the external D.-C. milliarnmeter.

The full scale indication of the meter 78 may next be set for which operat1on it is desirable that the oscillator 15 be operative w1th a vacuurn tube voltmeter or bridge of known accuracy connected to yack 34. Typically then and with switches 58 in the middle osition, the oscillator control potentiometer 43 is adjusted to where the external voltrneter connected to jack 34 indicates 15 volts. When this occurs the meter series resistance 71 is adjusted to where full scale deflection of meter 78 is obtained.

Frequency calibration of the oscillator variable condenser 24 may be made With a heterodyne frequency meter connected to jack 34. Inductance 17, although shown singly in Fig. 1 to avoid undue circuit complexity, normally would be band switched to cover a wide range such as 2-20 megacycles.

The voltrneter 78 itself is a simple, rugged, 1 milliampere full scale milliarnmeter. The Scale thereof is calilnated' directly in ohrns impedance to malte it direct rea.ding. In the typical instrumenfthus far descrilped, a full scale meter reading of 4.5 R. M. S. volts is obtained for the range with switches 58 and 59 in the toprnosiposition, and a full scale reading of 15 volts is to be obtained with the switches 58 and 59 in the mid-position. Note that this requirement sets the ratio 015 the resistors 72, 73, 74 and 75. Resistor 73 is equal to four and onehalf fifteenths of the sum of resistors 72 and 73. Likewise a similar ratio prevails for resistors 74 and 75. The scale of meter 78 is first temporarily calibrated in R. M. S. volts for the two ranges of 4.5 and 15 volts R. M. S. then permanently calibrated ance by the simple expedient of dividing the R. M. S. volts numerals by a factor of 3) 10 the standarilrileasuring current thereby giving seales of 1500 ohrris impaqk anee.

In operation of the apparatus thus far described, the unknown impedande is connected to the iack34 ai1d the oscillator plate voltage is adjusted by mearis of Potentiometer 43 to obtain a radio frequency current of 3 milliamperes in the unknown impedance as indicatecl on meter 35 With such adiustment the calibrated meter 78 directly indicates load impedance at the frequency of the oscillator. The oscillatorfrequeney may (bei: be varied mannally and readings of meter 78 rnacle to obtain impedance maxima and minima if the Ioad is of a resonant nature such as an .antenna or a mismatchcd transmission line.

In the case of a transrnission line this instrument my also be used to determine attenuation Per unit length cf cable employing the relation. 1

In this arrangement the where v a=attmuation per hundred feet Zo=characteristic impedance'of the Gable Zmu=maximum im1iedance at any resonant frequency L=Iength of cable in hundreds of feet T0 determine the electrical length cf a cable it is merely necessary to note the frequency of two successive. energy maxima, e1ther with the cable termx'nated in. an open circuit or in a closed circuit then,

where L'=electrical length in meters AF=difference in frequency betwcen two impedance maxima Knowing thus the electrical length of the cable, the phYsical]ength may be calculated from the relationshjp c where L=physical length of cable in meters L=electrical length of the cable in meters v=velocity of propagation in the cable =velcpcity of propagation in free space.

This same relationship may be used to determine the propagation constant for the cable 1'f that is nnknown and the physical length can be measured.

A van'ant embodiment of the features of the present reduce the danger that radio-frequency signals will mach the balancing tube 101 and associated circuits. tionally switches 103 and 104 are of the four position range, rather than requin'ng a compromise settx'ng as required in the apparatus cf Fig. I. Three series variable resistors 106, 107 and 108 are selectable one at a t1'rne by switch 105.

m the art and 1's partxcularly desirable bepause it dass n'ot require a tapped condenser 01' tapped inductance but uses an amph'fier tube for feedback. this circuit the inductance 109 and the capacitance 109-a are ganged with the unkmwn impedance. With care in parts layout and Wiring this shunt capacitance can be held to 2.5 micro micro-farads or lass but even this is noticeable at high impedances. The rectifier coupling capacitance 50 is consequently made very small and provides very loose Accordingly, to provide the 10W range (0-100 ohms), a capacitance 110 typwally cf .001 microfarads is manuofthe increased couph'ng thereby provnied, increases the sensitivity of the instrument. band-switched however it is not desirable that such an couple leads be electrically insulated from the circnit leads connecting jack 34 and fuse 115.

at a selected frequency.

2. Apparatus for determining the impedance of an impedance device compn'sing, generator means for producing radio frequency current, couph'ng means for applying This type oscilla-tor is Well known the radio frequency current to said impcdance dcvice, said generator means comprising -.an oscillator, anisolating ampllfier, and means for varying the frequency cf said oscillatorthermocouple means serially connected with saidimpedance device foimeasuring the current flow therethrough, voltage control means connected to said generator means controlling the flow of current through said impedance device and voltmeter means connected in parallel with said impedance device for providing a meter d eflection dependent upon the voltage developed 7 across said impedancc device with a predetermined eurrent flow therein, and a calibration.scale f0r said voltmetex: means co-operative with meter deflection thcrein 10 indicate impedance corresponding thereto.

3. Apparatus for detexmining the impedance of an impedance dev1ce comprising, generator means for producing radio frequency current, coupling means for applying the radio frequency current to said impedance device, said generator means comprising an oscillator, means ior varying the frequency of said oscillator, a voltmeter counected across said impedance device for measuring the impedance thereof, said yoltmeter comprising a pair of thermionic rectifying devices, a pair of amplifiers balancing means interconnecting one cf said rectii'ying devices with onc of said amplifiers for balancing said voltmeter, and switch rneans associated with said amplificrs for accommodating a plurality cf voltage ranges for said voltmeter. 1

4. The combinatlon according thermocouple tor and "impedance de\iice for measuring currentflow through said device, andc'ont'rol mans for adjusting the amplitude of current in said impedance device to mintain a selectedflow therein.

S. The combination according to claim 3 wherein v0ltage control means areconnected to said oscillator ior controlling the voltage output of said oscillator.

6. The combination according to claim 5 wherein said voltage con-trol means comprises a vacuum tube, and a potentiometer connected to the grid of said vacuurn tube for controlling the supply of voltage to said oscillator.

References Clted in the file of this patent UNITED STATES PATENTS 1917,417 Zuschlag July!ll, 1933 2,324,215 Kinsburg July 13, 1943 2,337,759 Loughlin Das. 28, 1943 2,406,405 Salisbury Aug. 27, 1946 2,535608 Smith Dec. 26, 1950 2588702 Cornelius Mm. 11, 1952 2,593175 Packard et a1. Apr. 1 5, 1952 to &:laim 3 Whetein means are connectgd between said oscillzx- 

